Matrix assisted ionization (MAI) is a general term used to describe a mass spectrometer ion source in which ions are formed by the interaction of an analyte molecule with specific matrix compounds that promote the formation of ions. As with matrix-assisted laser desorption ionization (MALDI), the matrix is mixed with the analyte and deposited and dried on a sample target. Ion formation is associated with the production of particles by laser ablation, mechanical shock, solvent boiling, or sublimation. Some matrix compounds that have been developed for MALDI can also be used for matrix-assisted ionization, but there are many compounds that are unique to MAI. Unlike MALDI, MAI tends to produce ions that are highly charged.
MAI has some potential advantages for mass spectrometry imaging due to its simplicity, low fragmentation, and tandem mass spectrometry facilitated by highly charged ion formation. For imaging in laser-spray mode, a pulsed laser is directed at a thin tissue section in transmission mode (back side irradiation) to create ions by MAI. Matrix-assisted ionization in vacuum (MAIV) can be used for the analysis of tissue by spotting matrix on selected areas and applying vacuum to the entire tissue section. Precision spotting can limit the exposed tissue area to several hundred μm. An alternative approach uses a glass melting point tube to sample from tissue under ambient conditions for MAI. Better temporal and spatial control of ion formation could add significant utility to these imaging approaches.
Precise control of material removal from metal sample surface for mass spectrometry analysis can be achieved using a locally directed shock pulse. Shock-generation of ions for MAI can be implemented in a number of ways. The simplest is to strike a target near the inlet of the mass spectrometer. Other methods for particle production include devices such as a pellet gun or mouse trap to produce a mechanical shock. Laser induced acoustic desorption (LIAD) uses a pulsed nanosecond laser that is directed in transmission geometry at a thin metal foil, which ejects material from the opposite side. Post-ionization can be accomplished using electron ionization, electrospray ionization, and photoionization. A similar approach that does not require a laser nebulizes liquid samples from piezoelectric ally driven targets using surface acoustic wave nebulization (SAWN), which uses a high frequency piezoelectric device to nebulize a thin film of liquid from a surface and bare ions are formed upon solvent evaporation and sampled into a mass spectrometer ion source.